1. Field of the Invention
The present invention relates to a transmitter for radio communications equipment which use a digital modulation system, such as a U.S. digital cellular system, having an amplitude modulation component and a phase modulation component for the purpose of supplying a stable burst-state transmission output.
2. Description of the Prior Art
Conventional radio communications equipment of this type generally use frequency modulation. One of the most typical examples of this conventional system is found in analog cellular telephone systems.
The transmitter in such frequency modulation radio communications equipment uses a feedback loop to control the transmitter output. In this feedback loop, a detector detects part of the transmitter output signal, which is amplified by a power amplifier; the detected signal is compared with a reference voltage signal by a differential amplifier, and a gain control terminal is controlled according to the resulting error signal. However, while this feedback loop control system is effective with frequency modulation systems in which an amplitude modulation component is not used, it cannot be used in digital modulation systems using both amplitude and phase modulation components. The feed-back loop functions to track and cancel the amplitude modulation component of the transmitter output signal, and is known to distort the modulated output signal. In addition, if the time constant of the feed-back loop is increased so that the amplitude modulation component is not tracked, a burst-state transmission signal that ramps up/down at high speed cannot be obtained.